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\begin{equation*}\partial^2 Ψ \over \partial x^2 \; + \partial^2 Ψ \over \partial y^2 \; + \partial^2 Ψ \over \partial z^2 \; = 1 \over v^2 \; \partial^2 Ψ \over \partial t^2 \;\end{equation*}
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